If it is intended as an inserted phrase, "some believe" should have a comma after it. ". However, in this case the comma after "died" is arguably not strong enough.
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park sang joonThank you, GPY, for your very helpful answer. Then, how about two enclosing dashesThe fact that the main wording does not read properly is a bit distracting when it comes to evaluating the punctuation possibilities, so I'll change that part.